Ask HN: Why do we not see non-tracking 'fallback' ads?

uBlock Origin blocks all the tracking ads, and this leaves empty space on many websites. Why don't the advertiser networks (Google/Meta/others) prepare 'fallback' ad content, and put that content into this space instead?

For example, my Twitter feed or Instagram stories loaded with uBlock origin have zero ads. I understand non-tracking ads are comparatively ineffective, but can't Meta insert fallback ads when their primary ad is blocked?

Even if there are technical limitations in doing this, from a high-level perspective, why is it not worth the effort for AdSense/Meta to solve these limitations, so they can deliver fallback ads to end users? Isn't some advert better than no advert at all?